Motion detectors often use passive infrared (PIR) detectors, microwave transceivers, or both to sense motion. A PIR motion detector has a lens for monitoring IR energy in its field of view. The field of view of the PIR is an unobstructed view perceived from the lens of the PIR detector. The lens focuses or shapes the field of view of the PIR. The PIR motion detector senses a change of IR energy (i.e., heat) received when a body having a temperature different from the ambient temperature enters the PIR's field of view. When the monitored IR energy changes (i.e., a change of temperature), the presence of the body is detected. Typically, a PIR detector detects the presence of a warm body, i.e., the body temperature of a person. However, the PIR detector will also detect the presence of a body having a temperature lower than the ambient temperature. For simplicity, this discussion refers only to warm bodies, but the reader understands that the discussion is equally applicable to cool bodies. The change in the IR energy is due to the temperature differential between the temperature of the warm body and the ambient temperature. The PIR may be connected to trigger an alarm, turn on lights, etc.
In addition to IR energy changes, detection of a warm body depends on other factors as well. One such factor is the optical gain of the lens, which in turn is a function of the focal length and the area of the lens. A larger lens area allows collection of more IR energy, resulting in a higher gain. The shorter the focal length, the wider the field of view and its cross-sectional area of coverage.
The size of the warm body in relation to the size of the beam is important. Typically, in order to produce an adequate signal to detect the warm body, the warm body must substantially fill the entire field of view. Otherwise, the warm body may not be detected. Detection is also a function of the optical gain of the lens used to focus IR energy onto the PIR detector. The optical gain is a measure of the lens segment's ability to collect infrared energy. The optical gain is typically selected so that a body substantially filling the field of view will be detected. The optical gain could also be selected for optimum detection at a desired distance from the PIR detector.
A commonly used lens for a PIR motion detector is a fresnel lens. FIG. 1 shows a conventional fresnel lens 10. The conventional fresnel lens 10 has concentric rings 15 which are separated by grooves 20. The center of the concentric rings 15 is called the optical axis 25 of the fresnel lens 10. The fresnel lens 10 is normally flat and may be inexpensively manufactured by molding or pressing in the same fashion as a musical record or compact disc. The fresnel lens may be manufactured from flexible plastic so that a curved opening of a housing containing the PIR detector may be covered with the flexible fresnel lens.
The grooves 20 which separate the concentric rings 15 allow the fresnel lens 10 to function as a convex lens, yet remain relatively flat. FIG. 2 shows a side view of the fresnel lens 10. The protruding surface 30 of the center ring 35 has a convex shape. The protruding surfaces 40 of the remaining rings 15 have a partially convex surface. The shape of the concentric ring is the same as the surface of a conventional convex lens at a corresponding position. Similar to a conventional convex lens, infrared rays that pass through the optical axis 25 do not refract, whereas rays that pass through the rest of the lens 10 refract toward the center of the optical axis 25. This refraction is a function of the focal length of the lens 10 and results in focusing the incident IR rays 55 onto the focal point 60. In a PIR motion detector system, a PIR detector element 65 located in a housing 70 is placed near the focal point 60 and the lens 10 (or an array of lenses as shown in FIG. 3) is placed covering an opening in the housing 70 in front of the PIR detector element 65.
A lens for a PIR motion detector may be an array of fresnel lenses for monitoring an area, such as a room, in discrete zones of coverage. FIG. 3 illustrates such a lens array 80. Each fresnel lens or segment 85 of the fresnel lens array 80 may be focused on a different portion of the monitored area. However, because each segment 85 of the fresnel lens array 80 is approximately the same distance from the PIR detector element 65, all the segments 85 have approximately the same focal length.
FIG. 4 illustrates a motion detector 100 mounted on a wall or in a corner of a room. The motion detector 100 includes at least one PIR detector element 65 enclosed in a housing 70. The fresnel lens 80, shown in FIG. 3, covers an opening in the housing 70 in front of the PIR detector element 65. As mentioned in connection with FIG. 3, each lens segment 85 focuses a different field of view onto the PIR detector element 65 which monitors the IR energy in these fields of view. Two of the fields of view are shown in FIG. 4. (To solve the problem of changes in ambient temperature due to, for example, sunlight warming a portion of a room, zones of coverage using adjacent fields of view are used. The fields of view are monitored by two PIR detector elements which create offsetting electrical signals if simultaneously activated. This discriminates between motion, which activates one element at a time, and ambient temperature, which activates both elements simultaneously. For simplicity, this discussion will refer only to one of the projected cross-sectional areas in a zone of coverage, but the reader understands that the discussion is equally applicable to a pair of horizontally adjacent projected cross-sectional areas defining a zone of coverage.)
Illustratively, one field of view 110, shown in FIG. 4, looks down with a large elevation angle 115 and protects a target area 120 near the motion detector 100. The field of view 110 has a projected cross-sectional area of coverage 120 at a small distance from the motion detector 100. Illustratively, the target area 120 is at a distance of approximately 10' from the motion detector 100.
The second field of view 130 looks down with a small elevation angle 135 and protects a target area 140 far from the motion detector 100. The second field of view 110 has a projected cross-sectional area of coverage 140 at a larger distance. Illustratively, the target area 140 is at a distance of approximately 40' from the motion detector 100.
The focal length is typically selected so that a human being substantially fills the projected cross-sectional area of coverage 140 of the field of view 130 at the furthest target area, in this illustrative example, 40'.
Because all the segments 85 (FIG. 3) of the lens 80 have approximately the same focal length, the projected cross-sectional areas of coverage 120, 140, at distances 10' and 40' respectively, have different sizes. As the distance from the lens 80 is decreased by half, the cross-sectional area is reduced to one quarter.
One problem which has been known to those skilled in the art is known as the rodent discrimination problem. Small animals or rodents R, such as rats and squirrels, entering the projected cross-sectional area of coverage 120 at a short distance, e.g., 10', from the lens 80, substantially fill the projected cross-sectional area of coverage 120 and are detected by the motion detector 100. A warm body completely filling the field of view may be detected. For simplicity, this specification refers to rodents. The reader should understand, however, that the invention is equally applicable to a relatively small, warm (or cool) body. At close range, a rodent R substantially filling the projected cross-sectional area of coverage 120 causes false alarms. However, the same rodent R will not be detected at a distance further away from the lens 80, e.g. 40', because the projected cross-sectional area of coverage 140 is larger and the rodent R does not fill the projected cross-sectional area of coverage 140.
FIG. 4 shows illustrative sizes of the projected cross-sectional areas of coverage 120, 140. As shown in FIG. 4, at a distance of approximately 40 feet from the lens array 80, the projected cross-sectional area 140 of the field of view 130 is approximately 1.3' by 2.6' for a lens array 80 having a focal length FL=1.2" and a 2 mm.times.1 mm PIR detector element 65. The field of view 110 for the same detector at a distance of approximately 10 feet from the lens array 80, has a smaller projected cross-sectional area 120 of approximately 0.3' by 0.6'. This small projected cross-sectional area 120 at 10' is approximately the size of the rodent R. Thus, the rodent R will substantially fill this small projected cross-sectional area 120 and provide adequate signal for detection by the PIR detector element 65 causing a false alarm.
One known prior art solution to this problem, known as "step optics", uses mirrors and is illustrated in FIGS. 5A and 5B. FIG. 5A shows a side view of a motion detector 300 using a mirror 302 having a stepped or staircase-like surface. The stepped mirror 302 is located inside the motion detector housing 70, behind the PIR detector element 65. The PIR detector element 65 is located at the focal point between the mirror 302 and a window 305. Each step segment of the stepped mirror 302 has a particular focal length and field of view which depend in part on the curvature of the mirror portion collecting IR energy from the field of view. The step segment 310, which monitors the distant field (at a small elevation angle), has a large focal length. By contrast, the step segment 320, which monitors the close in field (at a large elevation angle), has a small focal length.
FIG. 5B shows the fields monitored by the step segments 310, 320. The step segment 310, having the large focal length, monitors the distant field of view 130. The projected cross-sectional area 140 of the distant field of view 130 is large so as to allow a human being to substantially fill the projected cross-sectional area of coverage 140 at the maximum range, e.g., approximately 40', as discussed in connection with FIG. 4. The step segment 320, having the small focal length, monitors the close in field of view 330. The projected cross-sectional area 340 of the closer field of view 330, approximately 10' from the motion detector 300, is of similar size as the cross-sectional area 140 of the distant field of view 130. Therefore, a rodent R will not be detected, despite being near the motion detector 300, (e.g., at 10' from the motion detector 300 with a large elevation angle), since it will not substantially fill the projected cross-sectional areas of coverage 140, 340.
However, such a stepped mirror is complex and expensive to produce. Furthermore, the stepped mirror has an unpleasant appearance. The stepped mirror, however, is not visible because it is located inside the housing 70.
Thus, it is an object of the present invention to provide a lens for a PIR detector that can effectively discriminate against rodents yet is simple and inexpensive to manufacture.